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Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature, precipitation, and other atmospheric conditions, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.

Weather is driven by air pressure, temperature, and moisture differences between one place and another. These differences can occur due to the Sun's angle at any particular spot, which varies with latitude. The strong temperature contrast between polar and tropical air gives rise to the largest scale atmospheric circulations: the Hadley cell, the Ferrel cell, the polar cell, and the jet stream. Weather systems in the middle latitudes, such as extratropical cyclones, are caused by instabilities of the jet streamflow. Because Earth's axis is tilted relative to its orbital plane (called the ecliptic), sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (−40 °F to 104 °F) annually. Over thousands of years, changes in Earth's orbit can affect the amount and distribution of solar energy received by Earth, thus influencing long-term climate and global climate change.

Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes, as most atmospheric heating is due to contact with the Earth's surface while radiative losses to space are mostly constant. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Earth's weather system is a chaotic system; as a result, small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout history, and there is evidence that human activities such as agriculture and industry have modified weather patterns.

Studying how the weather works on other planets has been helpful in understanding how weather works on Earth. A famous landmark in the Solar System, Jupiter's Great Red Spot, is an anticyclonic storm known to have existed for at least 300 years. However, the weather is not limited to planetary bodies. A star's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind. (Full article...)

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Satellite image of Hurricane Dean approaching the Yucatán Peninsula
Satellite image of Hurricane Dean approaching the Yucatán Peninsula

Hurricane Dean was the strongest tropical cyclone of the 2007 Atlantic hurricane season. A Cape Verde-type hurricane that formed on August 13, 2007, Dean took a west-northwest path from the eastern Atlantic Ocean through the Saint Lucia Channel and into the Caribbean. It strengthened into a major hurricane, reaching Category 5 status on the Saffir–Simpson hurricane scale before passing just south of Jamaica on August 20. The storm made landfall on the Yucatán Peninsula on August 21 as a powerful Category 5 storm. It crossed the peninsula and emerged into the Bay of Campeche as a weaker storm, but still at hurricane strength. It intensified briefly before making a second landfall near Tecolutla, Mexico, on August 22. Dean drifted northwest, weakening into a remnant low which dissipated over the southwestern United States.

The hurricane's intense winds, waves, rains and storm surge were responsible for at least 45 deaths across ten countries, and caused estimated damages ofUS$1.5 billion. Dean's path through the Caribbean devastated crops, particularly those of Martinique and Jamaica. Upon reaching Mexico, Hurricane Dean was a Category 5 storm—the third-most intense Atlantic hurricane at landfall in recorded history. However, it missed major population centers, so it caused no deaths and less damage than its passage through the Caribbean islands as a Category 2 storm.

Dean was the first hurricane to make landfall in the Atlantic basin at Category 5 intensity in 15 years; the last storm to do so was Hurricane Andrew on August 24, 1992. Dean's landfall was far less damaging than Andrew's, but its long swath of damage earned its name retirement from the World Meteorological Organization's Atlantic hurricane naming lists.

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An atmospheric gravity wave manifests itself as altocumulus undulatus clouds in an arid environment, in the Tadrart Acacus region of southeast Algeria.

Recently selected pictures: Lightning striking the Eiffel Tower, Cyclone Catarina, Roll cloud, More...

More did you know...

...that the Flying river is the name given to the transport of water vapor from the Amazon rainforest to southern Brazil?

...that hurricane shutters are required for all homes in Florida unless impact-resistant glass is used?

...that the Joint Institute for Marine and Atmospheric Research is a combined weather and ocean research institute with the cooperation of the Office of Oceanic and Atmospheric Research and the University of Hawaiʻi?

...that the SS Central America was sunk by a hurricane while carrying more than 30,000 pounds (13,600 kg) of gold, contributing to the Panic of 1857?

...that a hurricane force wind warning is issued by the United States National Weather Service for storms that are not tropical cyclones but are expected to produce hurricane-force winds (65 knots (75 mph; 120 km/h) or higher)?

...that the Automated Tropical Cyclone Forecasting System is a software package for tropical cyclone forecasting developed in 1988 that is still used today by meteorologists in various branches of the US Government?

Recent and ongoing weather

This week in weather history...

July 17

2006: An active period of derechos began across the Midwestern United States and eastern Canada, killing several people over 5 days.

July 18

1986: Hurricane Estelle reached peak intensity over the open Pacific Ocean. Despite never directly impacting land, the storm would bring unusually rough surf to Hawaii that destroyed several homes and killed two people.

July 19

1996: Unprecedented rainfall started the Saguenay flood, killing at least seven people and causing as much as $1.5 billion (CAD) in damage in the Saguenay–Lac-Saint-Jean region of Quebec.

July 20

2008: Hurricane Bertha, the longest-lived July Atlantic hurricane on record, dissipated over Atlantic Canada.

July 21

1987: A rare high-altitude violent tornado downed around 1,000,000 trees in Yellowstone National Park and surrounding wilderness.

July 22

2006: Hurricane Daniel reached peak intensity over the open Pacific Ocean, with maximum sustained winds of 130 knots (150 mph; 240 km/h).

July 23

2012: Typhoon Vicente made landfall over Taishan, Guangdong, China. The storm killed 32 people, in addition to causing an environmental disaster when 7 shipping containers of polypropylene plastic pellets were washed into the sea near Hong Kong.

Selected biography

Portrait of John Park Finley in 1913

John Park Finley (April 11, 1854 – November 24, 1943) was an American meteorologist and Army Signal Service officer who was the first person to study tornadoes intensively and pioneer tornado forecasting. He also wrote the first known book on the subject as well as many other manuals and booklets, collected vast climatological data, set up a nationwide weather observer network, started one of the first private weather enterprises, and opened an early aviation weather school. (Full article...)

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The scope of WikiProject Weather is to have a single location for all weather-related articles on Wikipedia.

WikiProject Meteorology is a collaborative effort by dozens of Wikipedians to improve the quality of meteorology- and weather-related articles. If you would like to help, visit the project talk page, and see what needs doing.

WikiProject Severe weather is a similar project specific to articles about severe weather. Their talk page is located here.

WikiProject Tropical cyclones is a daughter project of WikiProject meteorology. The dozens of semi-active members and several full-time members focus on improving Wikipedia's coverage of tropical cyclones.

WikiProject Non-tropical storms is a collaborative project to improve articles related to winter storms, wind storms, and extratropical cyclones.

Wikipedia is a fully collaborative effort by volunteers. So if you see something you think you can improve, be bold and get to editing! We appreciate any help you can provide!

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